Bay, rumor has it that a wash with Fels Naptha soap right after being exposed (or suspected exposure) to PI will wash the oil away with no itching. Actually, washing with any good soap should cut the urushiol, but keeping Fels Naptha around is always a good idea as it also does a great job removing the PI oil from clothing in the wash.
Rubbing possible effected areas with denatured alcohol will dissolve the urushiol so take some along into the woods or PI areas with you. Actually any alcoholic beverage may be sacrificed to the PI gods, but then again you might want to reserve the good stuff for relief after the discomfort starts. The first 15 minutes or so after contact are when the first two options work best.
Fresh aloe vera plant gel can also be applied to the affected areas. Aloe's anti-inflammatory and antibacterial properties make it great for Pl. Finally, a few leaves from the common weed plantain, washed, mashed and applied as a poultice will help, but these last two are pretty much a.s.a.p. techniques to keep the PI problem down after the first 15 minutes.